1. Field of the Invention
This invention relates to molding processes and is more particularly concerned with foam articles and with processes for molding polymer foam articles.
2. Description of the Prior Art
It is well-recognized in the art of molding polymer foams, such as polyurethane, polyisocyanurate and the like foams, that relatively high pressures are generated by the expanding foam when the molding is carried out under "packing" conditions. The latter comprise charging to the mold an amount of polymer foam forming mixture in excess of that necessary to just fill the mold under free rise conditions.
Such packing conditions are employed particularly in the molding of parts of relatively high density i.e. of the order of 5 to 45 pcf or higher, for example, furniture parts and the like which are intended as substitutes for corresponding parts fabricated from wood, picture frames, decorative relief sculptures, grids for fluorescent lighting and the like.
In order to withstand the pressure developed in the mold and maintain the latter closed during the molding operation, it has become necessary to utilize molds constructed from material of high structural strength together with complex clamping mechanisms as well as auxiliary strengthening components such as jigs surrounding the mold. This has become particularly necessary in the case of the molding of articles of intricate design in which any distortion of the mold, by pressures created during the molding process, can result in bowing or distortion of the part. This is critical also in the case where it is necessary, as in the case of molding of a part containing a number of voids, that the two halves of the mold remain in sealing engagement not only at the outer edges but at a plurality of points in the interior of the closed mold.
For these reasons it has hitherto been common in the art to construct molds having a high degree of structural strength and this inevitably means that such molds are excessively heavy, particularly when fitted with necessary clamping devices and auxiliary strengthening devices. This in turn leads to complex equipment required to open and close the molds as well as to increased labor costs in running such operations on a commercial basis.
German Offenlegungsschrift No. 2,027,934, published Dec. 16, 1971, describes an attempt to reduce the complexity of equipment required to hold a mold closed during foaming by employing an inflatable bag as a part of the clamping mechanism. The arrangement shown in this reference does not, however, obviate the requirement to use mold parts having sufficient structural strength to resist deformation under pressure.
We have now found that the drawbacks inherent in the art processes described above can be overcome in a simple but elegant manner which will now be described.